An aircraft is propelled by several turbofans, each housed in a nacelle also accommodating a set of accessory actuation devices associated with its operation, such as a thrust-reverser device, and performing various functions when the turbofan is in operation or stopped.
A nacelle usually has a tubular structure comprising an air intake upstream of the turbofan, a mid-section designed to surround a fan of the turbofan, a downstream section accommodating thrust-reverser means and designed to surround the combustion chamber of the turbofan, and is usually terminated by an exhaust nozzle the outlet of which is situated downstream of the turbofan.
Modern nacelles are designed to accommodate a bypass turbofan capable of generating, via the air foils of the fan in rotation, a flow of hot air (also called the main flow) originating from the combustion chamber of the turbofan, and a flow of cold air (the bypass flow) which travels outside the turbojet through an annular passageway, also called the stream, formed between a fairing of the turbofan (or an internal structure of the downstream structure of the nacelle and surrounding the turbofan) and an internal wall of the nacelle. The two air flows are discharged from the turbofan through the rear of the nacelle.
Each propulsive assembly of the aircraft is therefore formed by a nacelle and a turbofan, and is suspended from a fixed structure of the aircraft, for example beneath a wing or on the fuselage, by means of a mast attached to the turbofan in its front and rear portion by suspension elements.
In such a configuration, it is the turbofan which supports the nacelle. Such an architecture is subjected to many combined external forces during the mission of the aircraft. Amongst other things this includes forces resulting from gravity, external and internal aerodynamic forces, gushes of wind, and thermal effects.
These stresses applied to the propulsive assembly are transmitted to the turbofan and cause deformations of the casings which directly impact the efficiency of the various stages of the turbofan. More particularly, in the case of a propulsive assembly called wasp-waist, that is to say having a long and relatively thin downstream portion relative to the intermediate structures and air intake, these stresses result in a particularly harmful deformation called “banana effect”, the downstream portion bending considerably.
Such a “banana effect” results in a deformation of the external structure of the nacelle formed by the various successive casings while the drive shaft, the blades of the fan and the internal blades of the turbofan remain rectilinear. The result of this is that the heads of the blades of the shaft come closer to the internal periphery of the casings. The general performance of the turbofan is thereby reduced relative to a configuration in which the casings sustain very little or no deformations, because it is then necessary to take account of this deformation in the design of the nacelle so as always to arrange a sufficient clearance between the heads of the blades and the periphery of the casings. This results in a portion of the supply air which is not compressed by the blades because it escapes through this considerable clearance.
A solution to this problem has been proposed in the as yet unpublished French patent application registered under number 06.05912 in the name of the applicant. FIGS. 1 and 2 of the appended drawing summarize the subject of this document FR 06.05912.
The nacelle 1 illustrated in these figures is called structural, that is to say that it supports the engine 7 and connects it directly to a fixed structure 2 of an aircraft via a mast 15 incorporated into its build. The rear section of the nacelle comprises an internal structural framework 18 consisting of radial frames 20 associated with uprights 22; longitudinal reinforcements 26, 27 associated with upper longitudinal structures 23 and lower longitudinal structures 24 complete this structure. In addition, a set of thrust-absorbing link rods 29 help with transferring the forces from the engine 7 to the fixed structure 2 of the aircraft. An aerodynamic and acoustic smoothing panel 21 is mounted on the framework 18 and surrounds the engine 7.
For a such a structural framework design to be able to be certified by the certification authorities and to be perfectly dimensioned with no other random reinforcement means, it is necessary to dissociate the acoustic panel from the transfer of the forces from the engine to the aircraft. In addition, the structure of the acoustic panel and the surrounding structure must not be too impacted if the pipework of the engine were to burst.